Question: If $3a + 7b + 2c = -3$ and $9x + 9y = 7$, what is $36x - 9a + 36y - 6c - 21b$ ?
Solution: $= -9a - 21b - 6c + 36x + 36y$ $= (-3) \cdot (3a + 7b + 2c) + (4) \cdot (9x + 9y)$ $= (-3) \cdot (-3) + (4) \cdot (7)$ $= 9 + 28$ $= 37$